Changes to weight painting in 2.81
It sounds like you might have had an issue with migrating a file from 2.80 to 2.81, but hopefully this helps someone else doing weight painting in 2.81.

In 2.81 with left click select, you should be able to select bones in Weight Paint mode with Ctrl-Left Mouse. You shouldn't need to switch to Pose mode to pose any bones.



For completeness, my full set of steps is:


Select the rig
Shift-select the mesh
Switch to Weight Paint mode
Use left click to paint + Ctrl-left click to select bones